A refractive index profile of an optical fiber is a graphical representation of the value of the refractive index as a function of optical fiber radius. Conventionally, the distance r to the center of the fiber is shown along the abscissa, and the difference between the refractive index and the refractive index of the fiber cladding is shown along the ordinate axis. The optical fiber refractive index profile is referred to as a “step” profile, a “trapezoidal” profile, a “parabolic” profile, or a “triangular” profile for graphs having the respective shapes of a step, a trapezoid, a parabola, or a triangle. These curves are generally representative of the theoretical or reference index profile (i.e., set profile) of the fiber. The fiber manufacturing constraints and stresses may lead to a slightly different profile.
An optical fiber typically includes an optical core, whose function is to transmit and possibly to amplify an optical signal, and an optical cladding, whose function is to confine the optical signal within the core. For this purpose, the refractive indexes of the core nc and the outer cladding ng are such that nc>ng. As is well known, the propagation of an optical signal in a single-mode optical fiber is divided into a fundamental mode (i.e., dominant mode) guided in the core and into secondary modes (i.e., cladding modes) guided over a certain distance in the core-cladding assembly.
Optical fibers are key components in modern telecommunication systems. Operators are constantly concerned about increasing the optical power transmitted along the fiber while limiting aging and losses of the optical fiber. For logistical reasons, operators are concerned about reducing the number of different kinds of fibers and are willing to use the same kind of fibers as feeder and termination fibers and as line fibers. Termination fibers need to have low bending sensitivity as they generally experience small bending radii in their installation. Feeder fibers need to have reduced Brillouin scattering as they distribute high input power into the telecommunication system.
One limitation for use of such optical fibers for telecommunication applications is loss due to stimulated Brillouin scattering (SBS). SBS is an optical nonlinearity due to interaction of optical photons with acoustic phonons of the glass matrix constituting the optical fiber. SBS limits the maximum optical power throughput of the optical fiber transmission system; as input power increases above what is known as the Brillouin threshold, the power that can be transmitted along the optical fiber reaches an upper limit. Any additional input power to the optical fiber scatters in the backward direction because of the interaction with acoustic phonons rather than propagating in the forward, launch direction as a higher power signal. Thus, SBS reduces the signal-to-noise ratio at the receiver and can cause the transmitter to become unstable as a result of the entry of reflected light. Moreover, the increasing use of optical amplifiers and solid state Nd:YAG lasers at ever increasing data rates over longer and longer distances all combine to exacerbate SBS.
Exemplary techniques suggested in the literature to increase the Brillouin threshold, minimize the detrimental effects of SBS, and increase the power handling capacity of the optical fiber rely on broadening either the photon energy spectrum of the source or the phonon energy spectrum of the glass to reduce the efficiency of the interaction. A broadening of the spontaneous Brillouin spectrum width will increase the Brillouin threshold. This can be achieved by making the Brillouin frequency shift to vary in the fiber section or along the fiber length.
European Patent No. 0839770 (and its counterpart U.S. Pat. No. 5,851,259) propose modulating drawing tension along the fiber to suppress SBS with no significant change in fiber loss or dispersion factors.
Japanese Patent Publication No. 09-311231 proposes changing the refractive index profile along the length of the fiber (i.e., axially) by varying the background fluorine concentration.
International Publication No. 2004/027941 proposes changing the refractive index profile along the length of the fiber by application of ultraviolet radiation or by thermal treatment.
Japanese Patent Publication No. 09-048629 discloses an optical fiber that includes a core region in which germanium dopant decreases from a central part to an outer periphery and fluorine dopant decreases from the outer periphery to the central part. The glass viscosity in the fiber cross section is therefore uniformly adjusted to prevent residual stress during fiber drawing.
Japanese Patent Publication No. 09-218319 discloses an optical fiber with reduced Brillouin scattering. The core diameter varies in the longitudinal direction of the optical fiber and includes a first dopant to increase refractive index and to lower the velocity of longitudinal acoustic waves and a second dopant to lower refractive index and to lower the longitudinal acoustic waves.
U.S. Patent Application Publication No. 2002/0118935 A1 proposes an irregular coating surrounding the optical cladding that varies in a lengthwise direction in order to alter the mode profile of the acoustic waves.
“Stimulated Brillouin Scattering Suppression by Means of Applying Strain Distribution to Fiber with Cabling,” N. Yoshizawa et al., IEEE JLT, Vol. 11, No. 10, pp. 1518-1522, (1993), proposes wrapping the fiber around a central rod to induce stress to change the energy distribution of acoustic phonons.
Some disadvantages of changing the index of refraction along the axial direction of the fiber, and tight fiber wrapping, include non-uniform fiber properties (e.g., splicing characteristics, Raman gain, and cut-off wavelength) along the fiber length and increased fatigue, which impacts optical fiber life.
U.S. Pat. No. 6,542,683 proposes broadening the energy spectrum of participating SBS phonons by providing a fiber core that includes alternating layers of glass-modifying dopant, which leads to non-uniform thermal expansion and viscosity profiles that impart a residual permanent non-uniform stress in the fiber section. At least two layers of differing coefficients of thermal expansion (CTE) and viscosities generate strain variation in the fiber section. This, in turn, generates Brillouin frequency shift variation, and hence linewidth increase of the mode.
Coefficients of thermal expansion and viscosity control in alternating layers are hard to achieve, and manufacturing processes capable of obtaining a preform of doped and undoped layers within the core requires costly equipment. Moreover, whenever the core is doped, fiber losses increase. This is especially so to the extent dopant concentrations have distinct variations (e.g., step-change variation). Such sharp variations will induce silica network defects at its interfaces, causing increased absorption loss of the fiber and degraded aging behavior.
U.S. Pat. No. 6,587,623 proposes controlling acoustic waves to be guided away from the portion of the waveguide that guides the light (i.e., guiding acoustic waves into the cladding) to reduce photon-phonon interaction and thus reduce SBS. Such an optical fiber is difficult to achieve, however, as the optical fiber refractive index profile must simultaneously satisfy good light guiding and bad acoustic guiding. In trying to reduce SBS in this way, drawbacks in optical transmission properties are expected.
“Effective Stimulated Brillouin Gain in Single Mode Optical Fibers,” J. Botineau et al., Electronics Letters, Vol. 31, No. 23, (Nov. 9, 1995), establishes that fibers possessing a trapezoidal refractive index profile achieve a higher Brillouin threshold compared to fibers possessing a step refractive index profile. Trapezoid profile shapes, however, might not be well suited for certain telecommunication applications.
U.S. Publication No. 2004/0218882 A1 discloses an optical fiber having a high SBS threshold. The core includes three regions with a specific doping scheme. The fiber refractive index profile disclosed in this document might not be well suited for certain telecommunication applications.
For compatibility between the optical systems of different manufacturers, the International Telecommunication Union (ITU) has established a standard referenced ITU-T G.652, which must be met by a Standard Single Mode Fiber (SSMF).
This G.652 standard for transmission fibers, recommends inter alia, a range of 8.6 microns to 9.5 microns for the Mode Field Diameter (MFD) at a wavelength of 1310 nanometers; a maximum of 1260 nanometers for the cabled cut-off wavelength; a range of 1300 nanometers to 1324 nanometers for the dispersion cancellation wavelength (denoted λ0); and a maximum chromatic dispersion slope of 0.092 ps/(nm2·km) (i.e., ps/nm2/km).
The cabled cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single mode after propagation over 22 meters of fiber, such as defined by subcommittee 86A of the International Electrotechnical Commission under standard IEC 60793-1-44.
Efforts to increase SBS threshold should not result in non-compliance with the G.652 standard.
Moreover, high optical power in transmission optical fibers may damage the fiber coating and thus accelerate aging of the optical fiber wherever bends are present. Reducing bending sensitivity of an optical fiber having a high Brillouin threshold would reduce aging problems of high power applications.
In addition, as previously noted, it is desirable to reduce bending sensitivity of optical fibers for use as termination fibers.
Typical solutions to reduce bending losses are to influence the MAC value. For a given fiber, the so-called MAC value is defined as the ratio of the mode field diameter of the fiber at 1550 nanometers to the effective cut-off wavelength λceff. The effective cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single mode after propagation over two meters of fiber such as defined by sub-committee 86A of the International Electrotechnical Commission under standard IEC 60793-1-44. The MAC value is used to assess fiber performance, particularly to achieve a compromise between mode field diameter, effective cut-off wavelength, and bending losses.
FIG. 1 depicts the experimental results that illustrate bending losses at a wavelength of 1625 nanometers with a bend radius of 15 millimeters in a SSMF fiber in relation to the MAC value at a wavelength of 1550 nanometers. FIG. 1 shows that the MAC value influences fiber bending and that these bending losses may be reduced by lowering the MAC value.
That notwithstanding, a reduction in the MAC value by reducing the mode field diameter and/or by increasing the effective cut-off wavelength may lead to noncompliance with the G.652 standard, making the optical fiber commercially incompatible with some transmission systems.
Compliance with the G.652 standard while reducing bending losses and increasing SBS threshold is a challenge for fiber applications in which single access optical fibers are to be used both in long-haul transmission systems and in Fiber-to-the-Home (FTTH) or Fiber-to-the-Curb (FTTC) systems.
“Bend-Insensitive and Low Splice-Loss Optical Fiber for Indoor Wiring in FTTH,” S. Matsuo et al., OFC 2004 Proceedings, Paper Th13 (2004), describes a refractive index profile for single mode fiber (SMF) that permits a reduction in bending losses. This disclosed fiber, however, shows a chromatic dispersion of between 10.2 ps/(nm·km) and 14.1 ps/(nm·km), which lies outside the G.652 standard.
“Low Bending Loss and Low Splice Loss Single Mode Fibers Employing a Trench Profile,” S. Matsuo et al., IEICE Trans. Electron., Vol. E88-C, No 5 (May 2005), describes an optical fiber having a central core, a first inner cladding, and a trench. Some of the exemplary fibers described in this document meet the criteria of the G.652 standard.
Enhanced Bending Loss Insensitive Fiber and New Cables for CWDM Access Networks” I. Sakabe et al., 53rd IWCS Proceedings, pp. 112-118 (2004), proposes reducing the Mode Field Diameter to reduce bending losses. This reduction in mode field diameter, however, leads to overstepping the G.652 standard.
“Development of Premise Optical Wiring Components Using Hole-Assisted Fiber,” K. Bandou et al., 53rd IWCS Proceedings, pp. 119-122 (2004), proposes a hole-assisted fiber having the optical characteristics of a SSMF fiber with reduced bending losses. The cost of manufacturing this fiber and its high attenuation levels (>0.25 dB/km) reduce its commercial viability in FTTH systems.
“Ultra-Low Loss and Bend Insensitive Pure-Silica-Core Fiber Complying with G.652 C/D and its Applications to a Loose Tube Cable,” T. Yokokawa et al., 53rd IWCS Proceedings, pp. 150-155 (2004), proposes a pure silica core fiber (PSCF) having reduced transmission and bending losses, but with a reduced mode field diameter that falls outside the G.652 standard.
U.S. Pat. No. 6,771,865 describes the refractive index profile of a transmission fiber with reduced bending losses. The optical fiber has a central core, an annular inner cladding, and an optical outer cladding. The annular cladding is doped with germanium and fluorine. U.S. Pat. No. 6,771,865 fails to disclose sufficient information to determine whether its disclosed fiber meets the G.652 standard.
U.S. Pat. No. 4,852,968 describes the profile of a transmission fiber having reduced bending losses. This disclosed fiber, however, has a chromatic dispersion that does not meet the G.652 criteria. The G.652 standard requires cancellation of chromatic dispersion at wavelengths of between 1300 nanometers and 1324 nanometers, but the fiber disclosed in U.S. Pat. No. 4,852,968 shows cancellation of chromatic dispersion at the wavelengths of between 1400 nanometers and 1800 nanometers.
International Application No. 2004/092794 (and its counterpart U.S. Pat. No. 7,164,835) describe the refractive index profile of a transmission fiber with reduced bending losses. The fiber has a central core, a first inner cladding, a second depressed inner cladding, and an outer optical cladding. Some of the exemplary fibers meet the criteria of the G.652 standard. The disclosed fiber is manufactured by Vapor-phase Axial Deposition (VAD) or Chemical Vapor Deposition (CVD). International Application No. 2004/092794 fails to identify the problems of microbending losses and Brillouin scattering.
In view of the foregoing, there is a need for a transmission fiber that meets the criteria of the G.652 standard (i.e., that can be used commercially in FTTH transmission systems of FTTH type) and that shows both reduced bending and microbending losses and increased stimulated Brillouin scattering threshold. Such optical fiber could be used as a single access fiber (i.e., a line fiber for long-haul transmission applications and a feeder fiber or a termination fiber in FTTH applications).
Furthermore, there is a need for an optical fiber having reduced bending losses and increased Brillouin threshold without unfavorably altering its fiber transmission characteristics (e.g., with limited fiber loss increase and without change to the fiber index profile).